Known in the past, for example, has been an extrusion press using copper, aluminum, an alloy thereof, etc. to extrude a tubular product by a direct double-action extrusion process. The extrusion press comprises a cylinder platen and an end platen arranged facing each other. The cylinder platen is provided with a main cylinder, main ram, extrusion stem, and mandrel, while the end platen is provided with a die. Between the extrusion stem and die, there is a container which can be made to freely advance and retract by container cylinders.
The extrusion stem has a dummy block arranged at its front end. The extrusion stem is attached to the main ram assembled in the main cylinder provided at the cylinder platen through the main cross-head. At the center position of the extrusion stem, the mandrel is arranged together with a piercer cylinder rod to be able to accompany and advance and retract with the extrusion stem. Further, the die is attached to the end platen facing the extrusion stem.
Between the extrusion stem and the die, the container is arranged to be able to advance and retract, in which a billet is held. The extrusion stem moves the billet stored in the container to the die side to thereby push the billet and complete the upset operation. After the upset operation, the mandrel advances to pierce the billet. The mandrel stops at a predetermined advancing position of the die. The extrusion stem is then again advanced to extrude the billet as a tubular product.
In this double-action extrusion press, when making the front end part of the mandrel stop at a predetermined position of a bearing part of the die and then extruding the product, the position of the mandrel is held so that its stopping position does not shift even if the relative positions of the mandrel and the bearing part of the die changes by a pulling action by the product.
PLT 1 discloses a double-action extrusion press which is provided with a piercer cylinder provided inside a main cylinder and a trigger forcibly connected with the mandrel away from the axial center of the extrusion press. This trigger acts on a hydraulic pilot valve to hold a bearing part of a die at a predetermined axial direction position (stopping position). For this, a certain amount of the pressurized fluid medium starts to be supplied to a rod side chamber of the piercer cylinder. Further, the position holding operation is controlled so that the amount of the pressurized fluid medium supplied matches the amount of increase of volume of the piercer cylinder rod side chamber when the mandrel is stationary and the main ram advances.
In this regard, in this conventional double-action extrusion press, the hydraulic pilot valve is switched mechanically through the trigger and a connecting rod to supply a certain amount of pressurized fluid medium to thereby hold the mandrel at a predetermined position of the bearing part of the die, so a delay occurs in control by exactly the amount of the stroke of movement corresponding to a land of a spool of the hydraulic pilot valve and a front end part of the mandrel moves back and forth by several millimeters with respect to the predetermined stopping position during an extrusion operation.
Furthermore, when changing the front end position of the mandrel or changing the extrusion speed, it is necessary to adjust the position of the trigger and the amount of fluid and pressure supplied to the rod side chamber of the piercer cylinder so as to adjust the pressure each time.
For this reason, the wall thickness of the extruded tubular products fluctuates and a stable quality of tubular products cannot be obtained.
Furthermore, in a conventional double-action extrusion press, there is the following problem: After pushing the billet in the container by the extrusion stem, then upsetting the billet and piercing the inside of the billet by a mandrel, then extruding it by a fixed mandrel, a frictional force occurs between the surfaces of the billet and mandrel and a pull force acts on the mandrel during extrusion. Due to this, the extrusion force acting on the die decreases by that amount, so it is not possible for the extrusion force to be effectively utilized at the start when the extrusion force is most required.